Because the demands on communications have recently increased, extension of a broadband transmission line is an urgent necessity.
As a part of the above extension, a high-quality optical transmission line for a DWDM signal of, for example, 10 Gbit/s (40 Gbit/s in future) to be the mainstream hereafter is used for long-distance-land and submarine-communication lines instead of a conventional optical transmission line for a WDM signal of, for example, 2.5 Gbit/s.
In general, a bit error rate (BER) is used as a measuring system for evaluating a digital communication line.
However, because the quality of a communication line is improved as described above, an existing BER measuring system has a problem that the system requires an extremely long time (e.g. tens of hours or more) for quality evaluation.
Moreover, because a conventional error-measuring system generates a pattern the same as the pattern of an optical signal to be measured inside, the system has problems that a configuration becomes more complex and the pattern of an optical signal to be measured must be known (e.g. PRBS).
Therefore, a quality evaluation technique using a Q-factor is proposed as an effective quality evaluation technique for a high-quality optical transmission line instead of a conventional error-measuring system (IEEE PHOTONICS TECHNOLOGY VOL. 5, NO. 3, MARCH 1993, pp. 304–306).
The quality evaluation technique using a Q-factor is realized by applying the concept of S/N defined in the field of analog to digital signals, which is an evaluation technique that assumes that noise is generated in accordance with a normal distribution, so-called Gaussian distribution.
That is, as shown in FIG. 8, when assuming the average value of ON levels as μ1 the standard deviation of ON levels as σ1 the average value of OFF levels as μ0, and the standard deviation of OFF levels as σ0 in the so-called eye pattern of an optical signal to be measured, a Q-factor is shown by the following expression.Q=|μ1−μ0|/(σ1+σ0)
Moreover, the quality evaluation technique using a Q-factor makes it possible to evaluate the quality of any high-quality line in a very short time of several-minutes.
The International Telecommunication Union (ITU-T) recommendation G. 976(April 1997) describes that the above quality evaluation method using a Q-factor is adopted for a test method to be applied to an optical-fiber submarine cable system and the like.
A configuration of a device for measuring a Q-factor specified by analyzing a linear signal is shown in FIG. 3 of the ITU-T recommendation.
An optical-signal autocorrelation-bit-error detection apparatus using an electrical-branch system is used as means for constituting the Q-factor measuring device.
The optical-signal autocorrelation-bit-error detection apparatus using an electrical-branch system converts an optical signal to be measured into an electrical signal and branches the electrical signal into two signals in accordance with an electrical technique so as to measure bit errors by using one of the two signals as a reference signal and the other of them as a noise detection signal and thereby, comparing both the signals with each other.
The autocorrelation-bit-error detection apparatus is advantageous in that because a reference signal and a noise detection signal are generated by an optical signal to be measured, it is unnecessary to previously know the pattern of the optical signal to be measured (e.g. PRBS) and generate a pattern the same as the pattern of the optical signal to be measured inside, and thereby the configuration is simplified.
FIG. 5 is a block diagram showing a configuration of the above conventional optical-signal autocorrelation-bit-error detection apparatus using an electrical-branch system.
The conventional autocorrelation-bit-error detection apparatus 1 first converts an optical signal to be measured modulated by a pulse signal into an electrical signal by a light-to-electricity converter (PD) 2.
Moreover, the electrical signal is branched into a first electrical signal and a second electrical signal by an electrical-branch unit 3.
Then, the auto-correlation-bit-error detection apparatus 1 decides the magnitude of the voltage of the branched first electrical signal and that of a reference voltage VREF by a decision circuit 4 and uses the decision result as a reference signal.
The decision circuit 4 includes a reference-signal generation circuit 5 in which the median of the amplitude of the branched first electrical signal is set as the reference voltage VREF.
Moreover, the autocorrelation-bit-error detection apparatus 1 decides the magnitude of the voltage of the branched second electrical signal and that of a noise detection voltage VTH by a decision circuit 6 and uses the decision result as a noise detection signal.
The decision circuit 6 includes a noise detection circuit 7 in which a voltage value at an optional level slid to the mark side (H side) or space side (L side) from the center of the amplitude of the branched second electrical signal is set as the noise detection voltage VTH.
Then,the autocorrelation-error detection apparatus 1 compares matching and mismatching between a reference signal obtained by the decision circuit 4 and reference-signal generation circuit 5 and a noise detection signal obtained by the decision circuit 6 and noise detection circuit 7 by a detector 9 including an error comparison circuit 8.
The detector 9 including the error comparison circuit 8 detects a case in which comparison results are not matched by assuming that autocorrelation bit errors occur in optical signals to be measured.
FIG. 6 is an illustration showing an autocorrelation bit error detection state of the above conventional optical-signal autocorrelation-bit-error detection apparatus 1 using an electrical-branch system.
As described above, because the reference voltage VREF is set to the median of the amplitude of the branched first electrical signal by the reference-signal generation circuit 5 included in the decision circuit 4, the voltage VREF is set to approximately midway between the mark side (L) and the space side (H) in FIG. 6.
As described above, the noise detection voltage VTH is set to an voltage value at an optional level slid to the mark side (H) or space side (L) from the center of the amplitude of the branched second electrical signal by the noise detection circuit 7 included in the decision circuit 6. In this case, however, a case is shown in which the voltage VTH is set to the mark side (H).
Moreover, FIG. 6 shows a case in which when the voltage of the branched first electrical signal is higher than the reference voltage VREF but it is lower than the noise detection voltage VTH, it is detected by the detector 9 of the autocorrelation-bit-error detection apparatus 1 that autocorrelation bit errors occur in optical signals to be measured.
FIG. 9 is an illustration for explaining the principle of a Q-factor measuring system to be executed by the above conventional autocorrelation-bit-error detection apparatus 1 using an electrical-branch system.
That is, as described above, the reference voltage VREF is set to the approximate center between eye patterns (“L” and “H”).
Moreover, the noise detection voltage VTH is changed between VTH1 corresponding to the average value μ1 of ON levels of the eye patterns and VTH0 corresponding to the average value μ0 of OFF levels of the eye patterns.
Thus, by performing the VTH1-and-VTH0-to-BER measurement, these measurement results are plotted as Pa1, Pa2, Pa3, Pa4, Pa5, Pa6, . . . , Pb1, Pb2, Pb3, Pb4, Pa5, Pb6, . . . as shown in FIG. 9.
Then, an optimum threshold voltage Vopt is calculated in accordance with the measurement results.
Then, the Q-factor and BER at the optimum threshold voltage Vopt are calculated.
The above calculation is performed in order to theoretically estimate a bit error portion in which a bit error occurrence rate including a V-shaped intersecting portion shown by a broken line below plots in FIG. 9 is as extremely low as, for example, the 10−20 order.
The relation between the Q-factor and the BER is shown by the following expression in accordance with ANNEX A A. 1 of the above ITU-T recommendation G. 976.BER={1/(2π)1/2}×{Exp(−Q2/2)/Q}
As shown in FIG. 5, the conventional autocorrelation-bit-error detection apparatus 1 using an electrical-branch system converts an optical signal to be measured into an electrical signal by the light-to-electricity converter (PD) 2 and then, branches the electrical signal into first and second electrical signals by the electrical-branch unit 3.
FIG. 7 is an illustration showing the conversion characteristic of the light-to-electricity converter (PD) 2, which is a graph in which the power of input light is expressed by the abscissa axis and the output current is expressed by the ordinate axis.
As shown in FIG. 7, in the conversion characteristic from light to electricity by the light-to-electricity converter (PD) 2, a portion in which linearity is shifted by 1 dB is assumed as a compression point Pcomp.
That is, the compression point Pcomp serves as a criterion for operating the light-to-electricity converter 2 in its linear area by avoiding its saturated area.
This is because when the light-to-electricity converter 2 is operated in its saturated area, it is impossible for the autocorrelation-bit-error detection apparatus 1 to accurately detect autocorrelation bit errors of optical signals to be measured.
Therefore, for the autocorrelation-bit-error detection apparatus 1 using the conventional electrical-branch system to detect autocorrelation bit errors of optical signals to be measured while securing linearity, it is necessary to input an optical signal having a power smaller than the compression point Pcomp as an optical signal to be measured.
Therefore, the autocorrelation-bit-error detection apparatus 1 using the conventional electrical-branch system has a problem that the power level of a usable optical signal is restricted to a predetermined level (e.g. 0 dB) or lower.
Moreover, in the case of the autocorrelation-bit-error detection apparatus 1 using the conventional electrical-branch system, because an electrical signal is branched by the rear-stage electrical-branch unit 3 of the light-to-electricity converter (PD) 2, the output power of the compression point Pcomp of the light-to-electricity converter (PD) 2 is further lowered due to branch and supplied to the decision circuits 4 and 6.
That is, because the power supplied to the decision circuits 4 and 6 is lowered due to the power loss based on a branch impedance in the electrical-branch unit 3, a problem occurs that the whole S/N of the autocorrelation-bit-error detection apparatus 1 is deteriorated.
For example, when an output of the light-to-electricity converter (PD) 2 is equal to 1 mW, inputs supplied to the decision circuits 4 and 6 are respectively lowered to 0.25 mW because the power loss of the electrical-branch unit 3 is equal to 0.5 mW.
Then, even if an electrical-amplifier is provided for the decision circuits 4 and 6 to amplify the lowered inputs, the electrical-amplifier has problems that it has a large noise (approx. 3 times larger than that of an optical amplifier in terms of NF), inferior linearity, and inferior response characteristic due to its waveform distortion.
Therefore, in the case of the autocorrelation-bit-error detection apparatus 1 using a conventional electrical-branch system, because it is difficult to improve the performance for measuring bit errors of optical signals to be measured in accordance with an autocorrelation system, the apparatus 1 has a problem that it is impossible to make the most use of advantages of the quality evaluation technique for a high-quality line according to the above Q-factor.